In the prior art, it is known to produce aluminum in coil form from a continuous casting apparatus wherein molten aluminum is delivered from a tundish and cast in the form of a metal sheet or strip and rolled into a coil on a coiler. Generally, in this process, molten aluminum is deposited on a moving chill surface from a tundish having an open outlet. An inlet is provided for the flow of molten metal into the tundish from a source of molten metal. The direct casting of the molten aluminum metal onto a chill wheel, preferably a grooved chill wheel, produces a cast aluminum product at a rapid rate. The aluminum cast strip is wound on a coiler in heated form, generally at a temperature in the range of about 900.degree. F.
Drag casting apparatus and methods of this type are described, for example, in U.S. Pat. Nos. 4,828,012, 4,896,715, 4,934,443, 4,945,974, 4,940,077 and 4,955,429. The disclosures of these patents are hereby specifically incorporated by reference with respect to the method and apparatus for the production of aluminum strip and coil from molten aluminum or aluminum alloys.
With reference now to FIG. 1, a continuous casting apparatus is illustrated which is typical of prior art continuous casting apparatus using a driven chill surface. The continuous casting apparatus is generally designated by the reference numeral 10 and is seen to include a tundish 1 positioned adjacent a driven chill surface 3. The chill surface 3 comprises the external cylindrical surface of a casting wheel 5. The casting wheel 5 is internally cooled with circulating water or other conventional cooling fluids to extract heat through the chill surface 3 so as to solidify molten metal 7 exiting the tundish 1.
The casting wheel 5 is supported by journal bearings 9 for rotation about a fixed horizontal axis. The journal bearings 9 are supported on the supporting frame 11 which supports both the bearings and the tundish 1. The casting wheel may be driven by a suitable drive means such as a variable speed motor and reduction gear mechanism, not shown, and a drive chain or belt 13 engaging the casting wheel 5.
After the molten metal 7 contacts the chill surface 3 and is solidified as a strip 15, a coiling apparatus 17 accumulates the strip in coil form for further processing.
The continuous casting apparatus 10 may also include a burner 19 to selectively apply heat to the chill surface at a location beneath the tundish 1. In addition, a top roll 21 may be provided which is uncooled or heated, the top roll being mounted for rotation in contact with the molten metal prior to complete solidification of the strip.
In order to properly coil the as-cast strip or further work the cast strip made by the apparatus of FIG. 1 into a product having a satisfactory quality, it is important to provide a cast strip leaving the casting surface having a proper shape or cross-sectional profile.
Difficulties have been encountered in prior art processes in achieving acceptable cast strip cross-sectional profiles in drag casting of aluminum products. Delivering molten aluminum from a tundish onto a moving chill roller surface produces a sheet product having an increased thickness at the edges thereof . This increased thickness is a result of a faster cooling rate at the edges of the chill surface and a corresponding "dog-bone" effect, or washboard or wavy edge. This condition prevents effective coiling of the cast strip as well as difficulties in further reducing the cast strip in subsequent rolling operations. Cold rolling of sheet or strip product generally requires that the sheet or strip have a slightly thicker center portion than edge portion. Strip having a "dog-bone" shape generally has thick edge portions and a thinner center section.
In the prior art, various devices have been proposed in conjunction with direct or continuous casting apparatus for improved continuous cast strip profile and gauge. In U.S. Pat. No. 4,828,012 to Honeycutt, III et al., a pair of risers or inserts are provided on the outlet edge of the tundish.
With reference now to FIG. 2, the risers as disclosed in U.S. Pat. No. 4,828,012 are illustrated. A pair of risers 23 and 25 are shown located on the top surface 27 of the tundish floor 29. Riser 23 is also adjacent sidewall 31 with riser 25 being adjacent sidewall 33.
The risers 23 and 25 are of generally rectangular configuration in both longitudinal and transverse cross-section so as to provide maximum-thickness at the point of intersection of the outlet 35 of the tundish 1. Each riser then tapers both longitudinally and transversely from the point of maximum thickness to smoothly blend into the top surface 27 of the floor 29.
It is disclosed in U.S. Pat. No. 4,828,012 that the risers 23 and 25 overcome the problem of "dog-bone" effect by increasing the thickness of the lip 35 of the tundish adjacent to the sidewalls 31 and 33 so as to reduce the contact time between the molten metal and the chill surface adjacent the marginal edges of strip.
U.S. Pat. No. 4,749,024 to Bartlett discloses another remedy to offset the natural tendency of direct cast strip to be thicker near the edges. In this patent, the tundish lip is contoured to provide a concave surface for the molten metal exiting the tundish. The object of the contoured tundish lip is to provide a sheet-casting process wherein the thickness profile along the sheet-length and across its width is controllable without changing the gap between the tundish and casting wheel.
U.S. Pat. No. 4,678,719 to Johns et al. discloses another continuous casting apparatus having particularly configured tundish sidewalls. The surfaces of the sidewalls of the tundish diverge to open upwardly to facilitate metal flow. The slight taper tends to improve metal flow from the exit end of the tundish.
U.S. Pat. No. 4,484,614 to Maringer discloses a continuous casting strip apparatus including a tundish for receiving and holding molten metal having a nozzle therein. The nozzle construction promotes rapid casting of metals with a minimum of metal turbulence during casting.
U.S. Pat. No. 4,685,505 to Truckner et al. discloses an improvement in roll casting apparatus using side edge dams for delivering a flow of gas toward the side edges of metal as it passes from a source of molten metal through a pair of opposing roll casting means. The non-contacting side edge dams contain the metal during roll casting from a nozzle tip.
U.S. Pat. No. 5,063,990 to Follstaedt et al. discloses a continuous casting apparatus with a particular weir design in the tundish to provide a more uniform flow of molten metal across the width of the tundish nozzle. The weir design includes a sloped rear wall and tapered sidewalls and a critical gap beneath the weir to reduce edge tearing and provide improved strip uniformity.
However, disadvantages still exist with prior art continuous casting apparatus. First, and with reference to FIG. 2, tundish edge risers fail to reduce the depth of the molten metal exiting the tundish sufficiently so as to cause rapid solidification of the molten metal contacting the chilled driven casting wheel surface. Thus, the molten metal leaks through the gap between the underside surface 37 of the sidewalls 31 and 33 and the casting surface 3. The molten metal then solidifies adjacent the runner surface 37 thus causing contact between the runner and the casting wheel and excessive vibrations during casting. Further, damage or erosion to the gasket between the sidewall and casting wheel surface results in premature shut-down of the casting apparatus. Contact between the surface 37 and wheel 3 also contributes to excessive vibration during casting. Finally, the leakage of molten metal also adversely affects the quality of the cast metal strip edge.
In view of the deficiencies in the prior art designs discussed above, a need has developed to provide improvements in control of molten metal exiting a tundish onto a driven casting wheel during continuous casting of metal strip product. In response to this need, the present invention provides a tundish outlet edge seal and riser which overcomes the disadvantages noted above in the prior art. The tundish outlet edge seal and riser extends outwardly from a tundish lip and along the tundish sidewalls while tapering toward the longitudinal axis of the tundish. None of the prior art discussed above teaches or fairly suggests a tundish riser which performs both a sealing function during continuous casting as well as reducing molten metal depth at the tundish sidewall outlet area to improve both the casting operation and cast strip quality.